The present invention relates to a magnetoresistive sensor, and more particularly, to a tunneling magnetoresistive (TMR) sensor and method of making the same.
Many different types of magnetoresistive (MR) sensors exist today, including anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), and tunneling magentoresistive (TMR) sensors. TMR sensors have proved to be especially attractive for high density data storage applications for various reasons including their large signal output and reduced shield-to-shield spacing. TMR sensors typically include a multi-layered portion called a TMR stack. The TMR stack includes a tunnel barrier layer positioned between two ferromagnetic layers. The tunnel barrier is a very thin electrically insulating layer, such as an aluminum oxide (Al2O3), while the two ferromagnetic layers are typically formed of an electrically conductive ferromagnetic material. The ferromagnetic layer on one side of the tunnel barrier is called the pinned layer. The pinned layer has a fixed magnetization direction and provides a reference direction for the TMR sensor. The ferromagnetic layer on the other side of the tunnel barrier is referred to as the free layer. The free layer has a magnetization direction that can rotate in response to an external magnetic field from the magnetic medium.
A sense current is supplied through the ferromagnetic layers and the tunnel barrier and flows perpendicular to the plane of the layers. While the tunnel barrier is an electrically insulating layer, a phenomenon known as spin dependent tunneling allows electrons to pass through the tunnel barrier at a rate related to the magnetization direction of the external magnetic field. As the external magnetic field from the magnetic medium causes the magnetization direction of the freely rotating ferromagnetic layer to rotate, the resistance of the tunnel barrier changes. This resistance is related to the difference between the magnetization directions of the two ferromagnetic layers. By measuring the change in resistance (for example, by measuring the current flow) the TMR sensor can read the flux from magnetic bits stored on the magnetic medium.
One of the problems manufacturers have faced in the formation of TMR sensors is the occurrence of defects such as pinholes in the tunnel barrier layer. These defects create small electrical connections, or ohmic shunts, through the tunnel barrier that allow electrons to flow through the tunnel barrier without tunneling. This ohmic electron flow through the pinholes is not related to the external magnetic field from the magnetic medium. Because of this, the defects decrease the signal to noise ratio, reduce the sensitivity of the TMR sensor, and change the resistance of the tunnel barrier.
Another problem that manufacturers have faced is that of thermal instability. TMR sensors experience changes in temperature during normal operation due to current flow through the TMR sensor, as well as external heat sources that warm the surrounding material. Some materials do not have good thermal stability, which results in an unwanted temperature dependence of the properties of the TMR sensor. One of these properties is the resistance of the material. It is desirable that the resistance of the barrier layer remain constant. However, some materials with poor thermal stability experience an undesirable degradation in the resistance of the TMR sensor over time as the temperature of the material changes.
Finally, materials chosen for the TMR sensor must have the appropriate properties to function in a TMR sensor. For example, the tunnel barrier must have an appropriate resistivity. In addition, the ferromagnetic materials must have the proper magnetic response properties, and must be compatible with the tunnel barrier material. Many materials do not have all of the desired properties for a TMR sensor, and therefore cannot be used as a TMR barrier material.
Therefore, there is a need in the art for a TMR sensor which resolves these and other problems in the art.